In recent years, highly efficient LEDs made with GaN-based semiconductors have become widely used in different technologies, for example in display devices, large electronic bill boards, street lights, car lights, and other illumination applications. LEDs are environmentally friendly, have a long lifetime, and low power consumption.
A conventional LED commonly includes an N-type semiconductor layer, a P-type semiconductor layer, an active layer, an N-type electrode, and a P-type electrode. The active layer is located between the N-type semiconductor layer and the P-type semiconductor layer. The P-type electrode is located on the P-type semiconductor layer. The N-type electrode is located on the N-type semiconductor layer. Typically, the P-type electrode is transparent. In operation, a positive voltage and a negative voltage are applied respectively to the P-type semiconductor layer and the N-type semiconductor layer. Thus, cavities in the P-type semiconductor layer and electrons in the N-type semiconductor layer can enter the active layer and combine with each other to emit visible light.
However, extraction efficiency of LEDs is low for many reasons. One reason is the typical semiconductor materials have a higher refractive index than that of air. Therefore, large angle lights emitted from the active layer may be internally reflected in the LEDs, so that a large portion of the lights emitted from the active layer will remain in the LEDs. Another reason is the current is limited under the P-type electrode, such that conduction of the current along a direction away from the P-type electrode is weakened. Thus, the lights emitted from the active layer are reduced. The extraction efficiency of LEDs is low so that the heat produced in the LEDs remains in the LED. Therefore, the property of the semiconductor materials is affected and the life span of the LED is shortened. As a result, the large-scale application of the LEDs is affected.
What is needed, therefore, is an LED which can overcome the above-described shortcomings.